Race, family history, and age are the unequivocally accepted risk factors for prostate cancer (PCa), as one of the most heritable cancers in western men. Androgen receptor (AR)-dependent signaling has a central role in neoplastic growth and progression of Prostate cancer. The incidence, mortality rate, and stage-specific aggressiveness of PCa in African-American men are higher than Caucasians. African-American men have higher levels of AR protein expression than the Caucasians both in benign (22% higher) and malignant (81% higher) prostate tissue. To date, this racial difference in AR expression remains one of the few possible explanations for the increased aggressiveness of PCa in African Americans. AR has therefore been suggested as a PCa susceptibility or predisposing gene. Genetic alterations in AR have been demonstrated to affect expression and activity of AR and its target genes during the progression of PCa, before or after androgen deprivation therapy. Molecular alterations in AR may allow for enhanced ligand affinity or decreased "off time" and these changes may stabilize AR and increase AR expression. If molecular changes in AR occur more frequently in African Americans, these changes may contribute to AR protein over-expression and, in turn, over-expression of AR-regulated genes that increase PCa growth. The search for germline risk alleles and their potential biological activities have been of major research interests aimed at explaining the hereditary basis and disproportionate racial distribution of PCa. We identified a novel germline missense mutation in the DNA-binding domain of AR-A1675T (Thr559Ser) in four patients of a high-risk African-American family with nine PCa-affected males in two generations. These data led us to hypothesize that the AR-A1675T mutation serves as a germline predisposing or risk allele with biological characteristics that favor prostate carcinogenesis in African-American men. This hypothesis will be tested by two specific aims. In Aim 1, we will determine the relative incidence of the AR-A1675T mutation and its attribution to familial PCa in African Americans. The analysis for the AR-A1675T mutation will be extended to 400 members of 40 high-risk African American and Caucasian families, each containing at least three men diagnosed with PCa and to a pool of 400 unrelated individuals from both ethnic cohorts who have not been diagnosed with PCa. Mutations will be identified using allele-specific oligonucleotide hybridization, restriction enzyme-based genotyping, and PCR- based automated sequencing. In Aim 2, we will determine the biological characteristics of the AR-A1675T mutation in AR-negative PCa cells or immortalized African American-derived prostate epithelial cells. The effect of the AR-A1675T mutation on growth, DNA-binding affinity, transcriptional activity and transactivation response to androgens, steroid hormones, and non-steroidal anti-androgens will be determined using methods routine in the laboratory. Significance: The results of this exploratory project combined with future large-scale studies including African American-targeted genome-wide association or expression array analysis might provide a better detection for population-attributable risk or prediction of the aggressive phenotype of PCa. PUBLIC HEALTH RELEVANCE: Race and family history are the widely accepted risk factors for prostate cancer (PCa). The incidence, mortality rate, and stage-specific aggressiveness of PCa in African-American (AA) men are significantly higher than in Caucasians. Our discovery of a novel germline AR mutation in a high-risk AA family might have predictive significance that would provide us a more effective diagnostic approach in AAs with familial PCa.